1. Field of the Invention
The present invention relates to aquagel-based concrete. More particularly, the invention relates to a lightweight, cellular, concrete which is prepared using aquagels. Methods of making the concrete and uses thereof are encompassed by the invention.
2. Description of the Art
Concrete is a composite material which is generally prepared using a material which forms a hard matrix (the cement or binder), a coarse granular material (the aggregate or filler), and water. Lightweight concretes can be categorized into ultra lightweight concretes useful for nonstructural purposes and structural lightweight concretes. ACI Committee 213 divides lightweight concrete into three categories on the basis of strength and unit weight: (1) low density, low strength concrete which is useful for insulation purposes; (2) moderate density, moderate strength concrete which is useful for concrete block and other applications where moderate strength is desired, and (3) structural lightweight concrete (Journal of the American Concrete Institute, 64:433-469 (1967)). [For a detailed discussion of lightweight concretes, see Concrete by S. Mindess and J. F. Young, Prentice-Hall, Inc. Englewood Cliffs, N.J. (1981), pages 581-596.]
Concrete density is generally related to the porosity (size and number of pores) within the concrete, including the porosity in the concrete matrix and porosity of the aggregate material, with the lowest density concrete generally having the highest porosity. One way to make low density concrete is to incorporate a lightweight aggregate. A problem associated with lightweight aggregate is its availability. Cinders from ash heaps at coal burning power plants have been used; however, with the decreased number of coal burning furnaces, kilns, and other facilities there is a limited supply of ash available. Other lightweight aggregates have been produced from slate, clays, shale, fly ash, or pumice, which are produced in kilns or sintering machines. These low density aggregates may function well; however, they are becoming increasingly expensive with increased material, fuel, and labor costs. Additionally, lightweight aggregate produced in kilns require expensive and cumbersome machinery and create air pollution which is undesirable. Dross is a byproduct of iron and steel production. Hot dross can be expanded by placing it in contact with water. Since dross is a byproduct, it is economical to use as a lightweight aggregate but it is not uniform in its properties which makes it difficult to achieve good quality control.
Another way to make low density, lightweight concrete has been to entrain gas or air or some type of an expanded particle as a portion or all of the aggregate. This has been carried out by various means, including incorporating air pockets into the concrete such as by whipping air into the concrete mix, by adding air-bubble-containing foam to a concrete mix, or by adding chemical substances which upon application of heat and/or reaction liberate a gas, e.g., foaming agents; or by incorporating expanded particles, such as polystyrene beads, into the cement. Some problems are associated with these technologies. For example, in the case of cellular concrete prepared using a foam containing small air bubbles, the concrete sets around the air bubble forming a concrete matrix with small voids or cells. Problems with this technique are that the air bubbles can escape from the concrete during mixing and pouring, the bubbles may coalesce forming large voids, the concrete may have non-uniform density throughout its profile with greater density near the bottom, the concrete must be poured shallow since hydrostatic pressure will compress the bubbles and affect concrete density, and the concrete may shrink and crack as it hardens. Additionally, to make cellular concrete requires special equipment. In the case of concrete prepared with polystyrene beads as a lightweight aggregate, one problem is that the beads tend to float to the surface of the concrete so it is difficult to get uniform density throughout the concrete. Additionally, polystyrene beads are made of plastic, a nonrenewable resource.
Further methods for preparing lightweight concrete or lightweight aggregate include using a colloidal solution or sol-gel composition of traditional aggregate materials such as bentonite, or by incorporation of polyethylene particles have been described. For example, U.S. Pat. No. 4,900,359 reports the preparation of a cellular concrete by adding a colloidal suspension of sodium bentonite, peptized calcium bentonite or attapulgite in water or a suspension of a gelled silica based sol-gel in water to a cellular concrete mix before the addition of an air-bubble foam composition including a foam-making agent. A problem with this technology is the same as that associated with bubble-foaming concrete mixes, namely, that the deeper the concrete is poured, the greater the hydrostatic pressure which can eventually compress the bubbles to only a fraction of their original volume. This hydrostatic pressure makes these concretes more dense at the bottom; the deeper the concrete, the greater is the problem.
U.S. Pat. No. 4,019,919 reports a full strength, reduced density concrete prepared by using polyethylene particles having irregular, roughened surfaces, as either part or all of the aggregate in the cement. The problem with polyethylene particles in concrete mixes is that plastic is less dense than concrete and tends to float to the surface of the concrete, thereby making it difficult to get uniform density. Further, polyethylene is typically derived from petroleum which is a non-renewable resource.